In order to produce a steel pipe having a desired strength, a heat treatment consisting of quenching and tempering is performed on the steel pipe during the production process thereof. When a steel pipe is quenched, a quenching method in which a heated steel pipe is immersed in a water bath for rapid cooling is often used, since such a method uses a large cooling capacity.
FIG. 1 is a schematic diagram showing an example of the process of immersing a heated steel pipe in a water bath. A quenching apparatus 1 shown in the same figure comprises a clamping device 5 for supporting a steel pipe 2, and a water bath 3. The clamping device 5 is made up of a first arm 6, and a second arm 7 which is swingably attached to the first arm. The first arm 6 includes a drive roller 61 and a roller 62 for supporting the steel pipe, and the second arm 7 includes a roller 71 for supporting the steel pipe.
When a heated steel pipe is immersed in a water bath by using a quenching apparatus shown in the same figure, the second arm 7 swings in a direction shown by an outlined arrow in the same figure, and the heated steel pipe is then placed on the drive roller 61 and the roller 62, which are included in the first arm 6. Thereafter, the second arm swings to return to a position shown in the same figure so that the heated steel pipe is rotatably supported by the drive roller 61 and the roller 62 included in the first arm and the two rollers 71 included in the second arm. While the steel pipe 2 is rotated in association with the rotation of the drive roller 61 (see the cross-hatched arrow in the same figure), the whole clamping device 5 swings as shown by the imaginary line in the same figure, thereby immersing the steel pipe in a water bath (see the diagonally shaded arrow).
The reason why the steel pipe is immersed in the water bath while being rotated is to prevent a partial decrease in strength for the steel pipe which has been quenched, which may occur when there is a difference in cooling rate between the water surface side and the water bath bottom side for the immersed steel pipe. In such an occasion, generally, a water flow is applied in an axial portion of the steel pipe to enhance the cooling effect of the steel pipe immersed in the water bath, and to uniformly cool the outer surface and the inner surface of the steel pipe.
FIG. 2 is a schematic diagram showing a conventional method for quenching a steel pipe, which is a process of generating a water flow in an axial portion of a steel pipe immersed in a water bath to rapidly cool the steel pipe. The same figure shows a water bath 3, a steel pipe 2 immersed in the water bath, and an axial center nozzle 8 disposed on the axis of the steel pipe. As shown in the same figure, by injecting cooling water to the axial portion at one end 2a of the steel pipe from the axial center nozzle 8, a water flow from one end 2a toward the other end 2b of the steel pipe is generated in the axial portion of the steel pipe (see the outlined arrow in the same figure). Hereafter, one end 2a of the steel pipe which is disposed near the axial center nozzle at the time of quenching is also referred to as a top end, and the other end 2b as a bottom end.
In a conventional method for quenching a steel pipe, by generating water flow in the axial portion of the steel pipe, the temperature of the inner surface of the steel pipe is prevented from becoming higher than that of the outer surface during rapid cooling, thus preventing the occurrence of a difference in strength between on the outer surface side and on the inner surface side of a steel pipe which has been quenched.
FIG. 3 is a diagram showing the relationship between the distance from the top end and yield strength in a steel pipe which has been quenched by a conventional method for quenching a steel pipe. In the same figure, the abscissa represents a distance (m) from the top end of the steel pipe, and the ordinate does a yield strength YS (MPa). The yield strengths shown in the same figure are those of a steel pipe which has been quenched by being heated and rapidly cooled. In rapid cooling of a heated steel pipe, a heated steel pipe is rotatably held by using a quenching apparatus equipped with a clamping device shown in FIG. 1 described above to be immersed in a water bath, and a water flow is generated in the axial portion of the steel pipe by the axial center nozzle disposed on the axis of the steel pipe shown in FIG. 2 described above. The steel pipe used for the quenching is made of carbon steel having strength corresponding to grade X65 of API standard, and has an outer diameter of 168.3 mm, a wall thickness of 18.52 mm, and a length of 12 m.
As shown in the same figure, in the conventional method for quenching a steel pipe, the yield strength declines on the bottom end side of the steel pipe compared with the top end side thereof. When the strength difference between on the top end side and on the bottom end side of the steel pipe increases, the product quality thereof will deteriorate, thus posing a grave problem.
Regarding the quenching method in which a heated steel pipe is immersed in a water bath to be rapidly cooled, various methods have been proposed including, for example, Patent Literatures 1 and 2. Patent Literature 1 has its objective to reduce the strength difference that occurs between on the top end side and on the bottom end side of a steel pipe which has been quenched, which is caused in such a manner that when a heated steel pipe is charged into a water bath with the axis thereof being kept in parallel with the water surface, buoyant force acts on the steel pipe due to air bubbles generated in the axial portion, and the bottom end tends to outcrop from the water surface, resulting in insufficient cooling. In the method for quenching a steel pipe according to Patent Literature 1, it teaches that a high-level progressive flow is formed by rapidly increasing the amount of water to be supplied to the water bath at the timing when the bottom end outcrops due to air bubbles, thereby increasing the water level in the area around the bottom end to prevent the bottom end of the steel pipe from outcropping from the water surface.
Further, Patent Literature 2 has its objective to solve a problem that flaws occur due to collision between the bottom end of the steel pipe and the wall surface of the water bath caused by a high-level progressive flow in the method for quenching a steel pipe according to Patent Literature 1. In the method for quenching a steel pipe according to Patent Literature 2, it teaches that by reducing the cross sectional area of the water bath on the bottom end side of the steel pipe, it is possible to reduce the amount of water necessary for forming a high level-progressive flow and to prevent the bottom end from outcropping from the water surface, deterring the axial movement of the steel pipe which is to be incurred by the water flow and to cause a collision between the bottom end and the wall surface of the water bath.
The quenching methods according to Patent Literatures 1 and 2, in which a heated steel pipe is immersed in a water bath, have their objectives to reduce the strength difference that occurs along a longitudinal direction of the steel pipe which has been quenched and is generated due to the outcropping of the bottom end of the steel pipe from the water surface during quenching. However, even when the quenching apparatus equipped with a clamping device shown in FIG. 1 described above is used and quenching is performed on the steel pipe without causing the bottom end side of the steel pipe to outcrop from the water surface, a difference in strength occurs between on the bottom end side and on the top end side of the steel pipe which has been quenched as shown in FIG. 3 described above.